My wife's students gave her a nice travel cup as a present but it is too big to fit into our car's cup holders. Instead of ditching a teacher gift we just built this travel mug adapter.
It's sized precisely for our cup and our car, but it's an OpenSCAD model! You can modify the sizes to fit your situation.
Print in two pieces and glue it together.
The ring around the bottom requires support.
The author marked this model as their own original creation.